Stir together 2 tablespoons of the sugar and the cornstarch into a bowl. Whisk in the egg yolks until well blended, thick and smooth.
In a heavy-bottomed nonreactive saucepan, combine the milk with the remaining 2 tablespoons sugar. Using the edge of a small, sharp knife, scrape the seeds from the vanilla bean into the milk and then add the bean. Place over medium heat and bring to a boil, whisking occasionally. Remove from the heat.
Whisking constantly, whisk almost one-third of the hot milk mixture into the egg mixture. Pour the combined mixture back into the hot milk mixture, whisk to combine, and return to medium-low heat. Cook, whisking constantly to keep the mixture from sticking and burning. Just before the mixture comes to a boil, it should thicken enough to coat the back of a spoon. As soon as the mixture boils, lower the heat slightly and continue to whisk for another 2 minutes to cook out the raw taste of the cornstarch and to allow the flavors to mellow.
Remove from the heat and strain through a fine-mesh sieve into a clean bowl. Cover with plastic film, pressing it directly on the surface to keep a skin from forming, and let cool to room temperature.
Place the chocolate in the top half of double boiler. Place over (not touching) gently simmering water in the bottom pan and heat, stirring occasionally, until completely melted. Using a rubber spatula, fold the hot chocolate into the cooled pastry cream. Cover with a piece of plastic film until ready to use. If not using immediately, refrigerate for up to 24 hours.